#!/usr/bin/python2

def range_score(score):
    if score < 60:
        return 'F'
    elif score <= 69:
        return 'D'
    elif score <= 79:
        return 'C'
    elif score <= 89:
        return 'B'
    elif score <= 100:
        return 'A'
def get_score():
    score = float(raw_input("Enter your score: "))
    while score < 0 or score > 100:
        score = float(raw_input("Error! Enter your score: "))
    print range_score(score)
if __name__ == '__main__':
    get_score()
    #print 'A'
